Preheat oven to 180°C.
Cook pasta as per packet instructions and keep warm.
Place cheese into the Thermomix bowl and mill 10 sec/speed 10. Set aside.
Place leek and butter into the Thermomix bowl and chop 4 sec/speed 5. Cook 5 min/Varoma/speed 1. Remove from bowl and set aside.
Make Bechamel and stir in half the cheese.
In a large bowl, combine cooked pasta, bechamel, cooked leek and tuna. Pour out into a baking dish. Top with panko and remaining cheese. Bake for 20 minutes. Then another 5 minutes on grill function to just brown the top.
Serve with a few dill sprigs and some fresh lemon wedges.